Redox-induced η⁵ ⇔ η³ haptotropy of the fluorenyl ligand in 9-substituted η⁵-fluorenylmanganesetricarbonyl complexes

Abstract

It has been shown by cyclic voltammetry in THF within the −90 to 40 °C temperature range that fluorenyl (η⁵-9-R-C₁₃H₈)Mn(CO)₃ complexes (R=Bu^t (3) and Ph (4)) undergo two-electron reduction to form allyl type [(η³-9-R-C₁₃H₈)Mn(CO)₃]²⁻ dianions as final products. At low temperatures complexes3 and4 are reduced in two one-electron steps according to the EEC-scheme. The first step is reversible and corresponds to the formation of 19ē-radical anions 3^−. and 4^−.. TheE ⁰ values for redox pairs3 ^0/−. and4 ^0/−. are −1.88 and −1.73 V, respectively. The further reduction of radical anions3 ^−. and4 ^−. at more negative potentials is accompanied by fast η⁵ → η³ haptocoordination of the fluorenyl ligand to form 18ē-dianions [(η³-9-R-C₁₃H₈)Mn(CO)₃]²⁻. These dianions obtained by the reduction of complexes3 and4 by the radical anion of pyrene are stable at −80 °C and are characterized by their IR spectra. At room temperature the η⁵ ⇔ η³ hapticity change is a fast and reversible process occurring at the step of 19ē-radical anions3 ^−. and4 ^−. and leading to the electron deficient 17ē-species [(η³-9-R-C₁₃H₈)Mn(CO)₃]^−., which are reduced easier than the initial complexes. As a result, complexes3 and4 are reduced to the corresponding dianions [(η³-9-R-C₁₃H₈)Mn(CO)₃]²⁻ at room temperature in one reversible two-electron step according to the ECE-scheme. Reactivities of 19e⁻-species of the isomeric η⁵- and η⁶-fluorenylmanganesetricarbonyl complexes are compared.